Fire resistant steel and method of production of same

ABSTRACT

The present invention provides first resistant steel superior in fire resistance having less variation in material quality and exhibiting a yield strength of ⅔ or more of that at ordinary temperature even at 600° C. and a method of production of the same, that is, fire resistant steel characterized by containing, by mass %, C: 0.01 to 0.03%, Mn: 0.2 to 1.7%, Si: 0.5% or less, Cu: 0.7 to 2%, Mo: 0.8% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, N: 0.006% or less, B: 0.0003 to 0.003%, V: 0.2% or less, Cr: 1% or less, Al: 0.1% or less, P: 0.03% or less, and S: 0.02% or less, containing Ni by mass ratio of Ni/Cu of 0.6 to 0.9, and comprising a balance of Fe and unavoidable impurities, and having a yield strength at 600° C. of 60% of the yield strength at 21° C.

TECHNICAL FIELD

The present invention relates to fire resistant steel used for structural members of buildings etc. and a method of production of the same.

BACKGROUND ART

Due to the increasingly greater number of stories of buildings, the increasing sophistication of building design technology, and the like, fire resistant designs have been reevaluated by a comprehensive project of Japan's Ministry of Construction. In March 1987, the “New Fire Resistant Design Act” was enacted. Under its provisions, the restriction under the old act requiring fire resistant protection to keep the temperature of a steel material at the time of a fire down to 350° C. or less has been scrapped. It has now become possible to determine a suitable method of fire resistant protection from the balance between the high temperature strength of the fire resistant steel and the actual load of the building. That is, when it is possible to secure a design high temperature strength of 600° C., fire resistant protection commensurate with that may be omitted.

To deal with these trends, the applicant previously proposed in Japanese Patent Publication (A) No. 2-77523 a building use low yield ratio steel and steel material superior in fire resistance and a method of production of the same. The gist of this prior application invention is to make the yield point at 600° C. 70% or more of that at ordinary temperature by adding Mo and Nb to improve the high temperature strength. The design high temperature strength of steel materials was set at 600° C. based in the discovery that this is the most economic in terms of the balance between the rise in cost of the steel materials due to the alloy elements and the costs of fire-resistant protection by the same. The H-section steel developed is characterized by reduction of the carbon content and addition of slight amounts of Nb, B, and Cu so as to produce a low carbon bainite structure and make the yield strength at 600° C. at least ⅔ the 440 MPa yield strength at ordinary temperature of the fire-resistant 590 MPa class standard, that is, 293 MPa, that is, to increase the high temperature strength. (The yield strength indicates the yield point when the yield point is clear and indicates the 0.2% yield strength when not clear).

Further, in addition to the above object, for the purpose of improving the brittleness at locations such as the fillets of H-section steel, Japanese Patent Publication (A) 9-137218 discloses building structure use H-section steel including Mo, Cu, and Ni so as to reduce the variation in material quality.

Furthermore, Japanese Patent Publication (A) No, 10-072620 discloses a method of production of H-section steel reduced in variation of material quality and superior in weldability.

DISCLOSURE OF THE INVENTION

The inventors etc. tried using steel materials produced by the above-mentioned prior application technology for various types of section steel, in particular the materials for H-section steel having severe restrictions in rolling due to their complicated shapes. As a result, it was learned that due to the differences in the rolling finishing temperature, rolling ratio, and cooling rate at the web, flanges, and fillet, the structure, in particular the bainite ratio, remarkably differs depending on the location of the steel material, the ordinary temperature and high temperature strengths, ductility, and toughness vary, and parts arise not meeting the standards of rolled steel materials for welded structures (JIS G3106) etc. In addition, the steel materials produced by said Patent Document 2 and Patent Document 3 suffered from surface defects due to the high temperature cracking by Cu or were inferior in fire resistance.

The present invention was made in consideration of this situation and has as its object the provision of fire resistant steel with small variation in material quality able to exhibit a yield strength of 60% or more of that at ordinary temperature even at 600° C. and a method of production of the same.

Therefore, the present inventors etc. engaged in research and as a result learned that in particular the addition of Cu is effective for causing the Cu which had entered into solid solution at an ordinary temperature to precipitate in the steel material structure at a high temperature and improve the yield strength at 600° C., but if the amount of Ni added to suppress high temperature cracking accompanying the addition of Cu becomes too great, precipitation of Cu at a high temperature becomes difficult and an improvement in the yield strength due to the precipitation of Cu can no longer be sufficiently achieved. The inventors engaged in further in-depth studies and as a result obtained the discovery that by including, by mass %, Cu in an amount of 0.7 to 2.0% and including Ni so as to give a mass ratio of Ni/Cu of 0.6 to 0.9, it is possible to enjoy both the suppression of high temperature cracking accompanying addition of Cu and the improvement of the yield strength due to precipitation of Cu with a good balance.

The present invention provides fire resistant steel characterized by containing, by mass %, C: 0.01 to 0.03%, Mn: 0.2 to 1.7%, Si: 0.5% or less, Cu: 0.7 to 2%, Mo: 0.8% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, N: 0.006% or less, B: 0.0003 to 0.003%, V: 0.2% or less, Cr: 1% or less, Al: 0.1% or less, P: 0.03% or less, and S: 0.02% or less, containing Ni by mass ratio of Ni/Cu of 0.6 to 0.9, and comprising a balance of Fe and unavoidable impurities, and having a yield strength at 600° C. of 60% of the yield strength at 21° C. Note that the yield strength indicates the yield point when the yield point is clear and indicates the 0.2% yield strength when not clear.

This fire resistant steel may further contain, by mass %, one or more of any of Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01% and comprise a balance of Fe and unavoidable impurities. Further, this fire resistant steel is for example section steel.

Further, the present invention provides a method of production of fire resistant steel characterized by heating a steel slab containing, by mass %, C: 0.01 to 0.03%, Mn: 0.2 to 1.7%, Si: 0.5% or less, Cu: 0.7 to 2%, Mo: 0.8% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, N: 0.006% or less, B: 0.0003 to 0.003%, V: 0.2% or less, Cr: 1% or less, Al: 0.1% or less, P: 0.03% or less, and S: 0.02% or less, containing Ni by mass ratio of Ni/Cu of 0.6 to 0.9, and comprising a balance of Fe and unavoidable impurities, to a temperature range of 200 to 1350° C., then starting rolling and, after finishing rolling, cooling by a cooling rate of an average of 0.1° C./s or more in the temperature range of 800° C. to 500° C.

This method of production may also be one heating a cast slab containing, by mass %, a further Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01% and comprising a balance of Fe and unavoidable impurities, to a temperature range of 1200 to 1350° C., then starting rolling and, after finishing rolling, cooling by a cooling rate of an average of 0.1° C./s or more in the temperature range of 800° C. to 500° C. Further, this method of production may be one producing for example section steel by rolling.

According to the present invention, it is possible to obtain high strength fire resistant steel giving a low carbon bainite structure supersaturated with Cu at ordinary temperature. The fire resistant steel of the present invention can be given a high temperature strength by precipitation of Cu by heating to 600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a rolling apparatus used in the examples of the present invention.

FIG. 2 is a cross-sectional view of H-section steel showing the position of obtaining a mechanical test piece.

FIG. 3 is a graph schematically showing ranges of the Ni/Cu ratio and the ratio of the yield strength at 600° C. (high temperature PS) and the yield strength at 21° C. (ordinary temperature YP) in the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the present invention will be explained in detail.

The high temperature strength of a steel material has substantially the same strengthening mechanism as that at ordinary temperature at a temperature of about ½ of the melting point of iron, that is, 700° C., or less, that is, is governed by 1. the increased fineness of the ferrite crystal particles, 2. the solution strengthening by the alloy elements, 3. the dispersion strengthening by the hard phases, 4. the precipitation strengthening by the fine precipitates, etc. In general, a rise in the high temperature strength is achieved by raising the precipitation strengthening due to addition of Mo and Cr and the softening resistance at a high temperature due to the suppression of elimination of dislocations. However, in low carbon bainite component-based steel with a carbon content of over 0.03%, island-like martensite was formed, the low temperature toughness remarkably fell, and parts not meeting the standards arose.

Therefore, in the present invention, the steel is made an extremely low carbon bainite component-based steel with a carbon content of 0.03% or less so as to increase the toughness by the suppression of formation of island-like martensite; further, to use the effect of improvement of hardenability by the composite addition of Nb and B so as to stably cause bainite transformation; and, at ordinary temperature, introduce Cu to the maximum extent in the α in solid solution and, at 600° C., make maximum use of the precipitation strengthening of Cu so as to achieve the targeted ordinary temperature strength, high temperature strength, and high toughness.

Here, in low carbon steel, the precipitation strengthening by Mo and V carbides which had been used in conventional fire-resistant steel cannot be expected to be effective, so the metal precipitating element Cu was employed.

Below, the reasons for limiting the ranges of ingredients and the rolling conditions in the present invention will be explained. Note that the ranges of ingredients are shown by mass %.

C: C is added for strengthening the steel. If less than 0.01%, the strength required as structural steel cannot be obtained, Further, excessive addition over 0.03% produces island-like martensite between the bainite laths and remarkably lowers the matrix toughness, so the lower limit was made 0.01% and the upper limit was made 0.03%.

Mn: Mn has to be added in an amount of 0.2% or more to secure the strength and toughness of the matrix, but the upper limit was made 1.7% in the allowable range of weld zone toughness, crack resistance, etc.

Si: Si is necessary for securing the matrix strength, preparatory deoxidation of the molten steel, etc., but if over 0.5%, hard structures of high carbon island-like martensite are formed inside the structures of the weld heat affected zone and the weld joint toughness is lowered. Therefore, the upper limit of the content of S was made 0.5% or less. Note that Si need not necessarily be included.

Mo: Mo is an element effective for securing the matrix strength and high temperature strength, but if over 0.8%, the hardenability will not rise and the matrix and weld heat affected zone will deteriorate in toughness, so the content was limited to 0.8% or less. Note that Mo need not necessarily be included.

Cu: Cu lowers the transformation point and increases the ordinary temperature strength. Further, the Cu not precipitating in the bainite transformation and becoming supersaturated enters into solid solution in the structure at ordinary temperature. A Cu phase precipitates on dislocations introduced by bainite transformation at the time of heating to the usage temperature of fire-resistant steel, that is, 600° C. This precipitation hardening increases the yield strength of the matrix. However, for this precipitation of the Cu phase in α, if less than 0.7%, this is in the solubility limit of Cu in α, so no precipitation will occur and therefore the above-mentioned strengthening will not be obtained. Further, if over 2%, this precipitation strengthening will become saturated and the toughness will be reduced, therefore Cu was limited to 0.7 to 2%.

Nb: Nb forms Nb carbonitrides and thereby immobilizes the C and N, suppresses the formation of boron compounds. Boron compounds promoting the formation of ferrite nuclei, and maintains the solid solution state of B. Further, solute Nb delays the grain growth of ferrite due to the drag effect, so even at a relatively slow cooling rate, holds the untransformed γ until the bainite transformation point and contributes to the stable formation of bainite. For this reason, the content is made 0.01% or more. Further, solute Nb obstructs movement of dislocations at a high temperature due to the drag effect and contributes to securing a high temperature strength. Therefore, to improve the high temperature strength, addition of 0.05% or more is preferable. However, if over 0.3%, the effect becomes saturated, so from the viewpoint of economics, the content was limited to 0.3% or less.

N: N forms B nitrides and assists the formation of ferrite, so the content of N was limited to 0.006% or less.

Al: Al is added for deoxidizing the molten steel and immobilizing N as AlN. If over 0.1%, alumina is formed and a drop in fatigue strength is caused, so the content was made 0.1 or less. Note that Al need not necessarily be included.

Ti: Ti is added to reduce the solute N due to the precipitation of TiN, suppress the precipitation of BN due to the increased fineness of γ grains, increase the solute B, and increase the effect of raising the hardenability by B. Due to this, it raises the ordinary temperature and high temperature strength. Therefore, if less than 0.005%, the amount of precipitation of TiN becomes insufficient and these effects are not exhibited, so the lower limit value of the amount of Ti was made 0.005%. However, if over 0.03%, the excessive Ti precipitates as TiC. This precipitation hardening degrades the toughness of the matrix and the weld heat affected zone, so the content was made 0.03% or less.

B: B raises the hardenability with slight addition and contributes to the rise in strength. However, if less than 0.0003%, the effect is not sufficient, while if over 0.003%, iron-boron compounds are produced and reduce the hardenability. Therefore, the B content was limited to 0.0003 to 0.003%.

Ni: Ni prevents high temperature cracking at the time of rolling accompanied with addition of Cu, so Ni giving a Ni/Cu ratio of 0.6 or more has to be included. On the other hand, Ni raises the solubility limit of Cu and decreases the amount of precipitation of Cu, so to secure high temperature strength, Ni giving an Ni/Cu ratio of 0.9 or less is included.

Cr: Cr improves the hardenability and thereby is effective for strengthening the matrix. However, excessive addition over 1% is harmful from the viewpoints of the toughness and hardening, so the upper limit was made 1%. Note that Cr need not necessarily be included.

V: V, by addition in a fine amount, enables the rolled structure to be made finer grained and strengthens it by precipitation of V carbonitrides, so enables a reduction of the use of alloys and improvement of the welding properties. However, excessive addition of V causes hardening of the weld zone and an increase in the yield point of the matrix, so the upper limit of the content was made V: 0.2%. Note that V need not necessarily be included.

Mg: Mg is preferably added for the purpose of finely dispersing the inclusions by making the oxides finer and forming sulfides. The amount of Mg is limited to 0.0005 to 0.01% because Mg is a powerful deoxidizing element. The precipitated Mg oxides easily float up and separate in the molten steel, so addition over 0.01% does not increase the yield, so the upper limit was made 0.01%. Note that the Mg alloys used for addition of Mg are for example Si—Mg and Ni—Mg. The reason for using an Mg alloy is that alloying lowers the concentration of Mg, suppresses reaction at the time of forming the Mg oxides, secures safety at the time of addition, and raises the yield of Mg.

Ca, REM: Ca and REM are preferably added to control the shapes of the sulfides and oxides. Ca was limited to 0.0005 to 0.005% and REM to 0.0005 to 0.01% because below these lower limit values, sulfides and oxides were insufficiently formed by these elements, while over the upper limit values, the oxides became coarse and the toughness and ductility dropped. Therefore, these elements were limited to these ranges.

The P and S included as unavoidable impurities are not particularly limited in amounts, but cause weld cracks due to solidification segregation and a drop in toughness and therefore should be reduced as much as possible. The amount is preferably an amount of P of 0.03% or less and an amount of S of 0.02% or less.

The cast slab having this composition was heated to give a surface temperature of the temperature range of 1200 to 1350° C. The heating temperature was limited to this temperature range since to produce section steel by hot working, heating to 1200° C. or more is necessary for facilitating plastic deformation. Further, it is necessary to cause the V, Nb, and other elements to sufficiently enter solid solution. Therefore, the lower limit of the heating temperature was made 1200° C. The upper limit was made 1350° C. from the viewpoints of the performance of the heating furnace and economy.

Further, in the case of extremely thick section steel of over 40 mm, if the cooling rate after the end of rolling becomes too slow, a large amount of a-structures will form in the structure. At the time of cooling, Cu will precipitate in the α and the amount of solute Cu at ordinary temperature will be reduced. In this case, the ratio of bainite will fall along with the formation of the α-structures, but precipitation strengthening does not usually raise the tensile strength as much as raising the yield point, so leads to a rise in the yield ratio (YR) and ends up reducing the earthquake resistance. Further, if the amount of solute Cu at ordinary temperature falls, an increase in yield strength due to the precipitation strengthening of the Cu phase at the time of heating to 600° C. can no longer be expected. This being so, the yield strength at 600° C. ends up falling below 60% of the yield strength at 21° C. Therefore, to secure sufficient bainite structures and increase the amount of solute Cu as much as possible at the time of cooling, the average cooling rate at the temperature range of 800 to 500° C. was made 0.1° C./s or more.

In the thus produced fire resistant steel of the present invention, due to the alloy design based on addition of slight amounts of Nb and B and high addition of Cu, the Cu will not precipitate much at all and will remain in a supersaturated state at the bainite transformation, while the Cu which had been in solid solution at ordinary temperature will precipitate in the steel structure and improve the yield strength at 600° C. at the time of heating this to 600° C. In this way, the fire resistant steel of the present invention has superior fire resistance enabling it to exhibit a yield strength of 60% or more of that of ordinary temperature even at 600° C.

The fire resistant steel of the present invention can be realized as steel suitably used for structural members of buildings etc. such as H-section steel, I-section steel, angle steel, channel steel, unequal side, unequal thickness angle steel, and other various types of section steel, thick-gauge plate and other such steel plate, etc. For example, when producing H-section steel as one example of the fire resistant steel of the present invention under the above examples, the steel has a low carbon bainite structure made finer at ordinary temperature and thereby exhibits substantially uniform mechanical properties at the different parts of the web, flanges, and fillet. The steel has sufficient strength and toughness even at parts of ½ the flange thickness and ½ the width where the mechanical test properties of H-section steel are hardest to guarantee. Further, at the time of heating to 600° C., fire resistance is exhibited due to the precipitation strengthening of Cu, so the result becomes high strength fire resistant rolled H-section steel superior in fire resistance and toughness. This H-section steel is superior in high temperature properties, so when used for fire-resistant steel for buildings, a protective thickness of 20 to 50% the conventional thickness enables sufficient fire resistance to be achieved. In this way, section steel having a superior fire resistance and toughness can be produced by rolling, a reduction in installation costs and a shortening of the work period and a resultant great reduction in costs can be achieved and an improvement in reliability of large buildings, securing of safety, and improvement of economy etc. can be achieved.

EXAMPLES

Below, examples will be used to show the effects of the present invention further.

The materials were melted in a converter, alloys were added, then Ti and B were added and the mixtures were cast by continuous casting into 240 to 300 mm thick cast slabs. The cooling of the cast slabs was controlled by selection of the amount of water in the secondary cooling zone below the molds and the pullout rates. The chemical ingredients of the steel types used in the examples are shown in Table 1. Steel Types 1 to 17 are in the range of the present invention, while Steel Types 18 to 38 are comparative steels outside the range of the present invention.

TABLE 1 (mass %) Steel type C Si Mn P S Cu Ni Cu/Ni Cr Mo Inv. 1 0.02 0.15 0.81 0.012 0.004 0.80 0.50 0.63 — 0.38 steel 2 0.02 0.15 0.50 0.011 0.005 0.88 0.60 0.68 — 0.31 3 0.02 0.16 0.80 0.011 0.005 0.98 0.70 0.71 0.07 0.30 4 0.02 0.15 0.95 0.012 0.005 0.99 0.62 0.63 — 0.31 5 0.02 0.15 0.98 0.010 0.005 1.01 0.61 0.60 — — 6 0.01 0.15 0.97 0.011 0.005 0.98 0.80 0.82 — — 7 0.03 0.15 0.98 0.011 0.004 0.90 0.70 0.78 — — 8 0.02 0.16 0.99 0.011 0.005 0.99 0.71 0.72 0.02 — 9 0.02 0.15 0.95 0.010 0.004 1.00 0.70 0.70 — — 10 0.02 0.16 0.91 0.012 0.005 0.95 0.76 0.80 — — 11 0.02 0.15 0.95 0.011 0.004 0.80 0.65 0.81 — — 12 0.02 0.30 0.30 0.012 0.005 1.98 1.66 0.84 — — 13 0.02 0.15 1.60 0.010 0.005 0.80 0.60 0.75 — — 14 0.03 0.15 1.65 0.010 0.005 0.75 0.46 0.61 0.80 — 15 0.02 0.16 0.80 0.011 0.005 0.98 0.70 0.71 — 0.30 16 0.02 0.15 0.95 0.012 0.005 0.99 0.62 0.63 — 0.31 17 0.02 0.15 0.98 0.010 0.005 1.01 0.61 0.60 — 0.30 Comp. 18 — 0.15 1.60 0.010 0.005 0.80 0.60 0.75 — 0.30 steel 19 0.04 0.16 0.80 0.012 0.004 0.98 0.65 0.66 — 0.30 20 0.02 0.60 0.94 0.012 0.005 0.97 0.60 0.62 — 0.29 21 0.02 0.25 0.10 0.012 0.005 1.88 1.62 0.86 — 0.59 22 0.02 0.18 1.81 0.012 0.005 0.78 0.61 0.78 — — 23 0.02 0.16 1.57 0.010 0.004 0.30 0.20 0.67 0.02 0.29 24 0.02 0.15 0.99 0.010 0.005 3.00 2.00 0.67 — — 25 0.02 0.15 0.95 0.010 0.004 0.81 0.40 0.49 — 0.20 26 0.02 0.15 0.81 0.011 0.005 1.60 0.80 0.50 — — 27 0.02 0.15 0.60 0.010 0.004 1.00 0.95 0.95 — — 28 0.02 0.15 0.50 0.011 0.005 1.02 1.01 0.99 — — 29 0.02 0.15 0.95 0.011 0.005 0.95 0.68 0.72 1.10 0.20 30 0.02 0.16 0.99 0.012 0.004 0.72 0.49 0.68 — 1.00 31 0.02 0.15 1.01 0.011 0.005 0.99 0.86 0.87 — 0.40 32 0.02 0.15 0.98 0.010 0.005 1.00 0.65 0.65 — 0.38 33 0.02 0.15 0.81 0.012 0.004 0.80 0.50 0.63 — 0.38 34 0.02 0.15 0.81 0.012 0.004 0.80 0.50 0.63 — 0.38 35 0.02 0.15 0.94 0.012 0.005 0.97 0.62 0.64 — 0.30 36 0.02 0.16 0.98 0.013 0.005 0.95 0.62 0.65 — 0.29 37 0.02 0.20 1.50 0.015 0.005 0.99 0.71 0.72 0.03 — 38 0.01 0.35 1.01 0.014 0.004 0.96 0.77 0.80 — 0.26 Steel type Nb V Ti Al N B REM Ca Mg Inv. 1 0.05 — 0.020 0.017 0.0027 0.0011 — — — steel 2 0.05 0.02 0.010 0.016 0.0037 0.0012 — — — 3 0.05 — 0.010 0.016 0.0033 0.0011 — — — 4 0.05 — 0.010 0.017 0.0033 0.0011 — — — 5 0.05 — 0.010 0.017 0.0035 0.0011 — — — 6 0.05 — 0.012 0.005 0.0037 0.0010 — — — 7 0.02 — 0.012 0.006 0.0038 0.0009 — — — 8 0.03 0.05 0.011 0.005 0.0039 0.0010 — — — 9 0.05 0.15 0.010 0.010 0.0045 0.0004 — — — 10 0.13 0.01 0.011 0.010 0.0040 0.0009 — — — 11 0.28 — 0.011 0.016 0.0037 0.0010 — — — 12 0.06 — 0.011 0.009 0.0029 0.0008 — — — 13 0.06 — 0.010 0.012 0.0035 0.0010 — — — 14 0.02 — 0.009 0.010 0.0055 0.0003 — — — 15 0.05 — 0.010 0.016 0.0033 0.0011 0.005 — — 16 0.05 — 0.010 0.017 0.0033 0.0011 — 0.008 — 17 0.05 — 0.010 0.017 0.0035 0.0011 — — 0.0090 Comp. 18 0.06 — 0.010 0.012 0.0035 0.0015 — — — steel 19 0.02 — 0.010 0.020 0.0029 0.0012 0.006 — — 20 0.05 — 0.010 0.015 0.0033 0.0010 — — — 21 0.03 — 0.010 0.015 0.0038 0.0012 — — — 22 0.04 — 0.014 0.016 0.0038 0.0011 — — — 23 0.02 0.02 0.010 0.030 0.0033 0.0010 — 0.016 — 24 0.02 — 0.010 0.020 0.0037 0.0003 — 0.008 — 25 0.05 — 0.011 0.009 0.0037 0.0010 — — — 26 0.02 — 0.012 0.010 0.0035 0.0011 — — — 27 0.04 — 0.012 0.011 0.0030 0.0012 — — — 28 0.05 — 0.010 0.016 0.0037 0.0010 — 0.010 — 29 0.05 — 0.011 0.015 0.0035 0.0011 — — — 30 0.03 — 0.012 0.013 0.0038 0.0008 — — — 31 0.04 0.3 0.013 0.016 0.0040 0.0015 — — — 32 — — 0.010 0.017 0.0035 0.0011 — 0.005 — 33 0.05 — 0.002 0.017 0.0027 0.0011 — — — 34 0.05 — 0.033 0.017 0.0027 0.0011 — — — 35 0.05 — 0.010 0.150 0.0039 0.0009 — — — 36 0.05 — 0.010 0.015 0.0100 0.0011 — — — 37 0.05 — 0.010 0.016 0.0033 0.0001 — 0.020 — 38 0.04 0.04 0.011 0.017 0.0036 0.0040 — — —

A cast slab of each of the steel types shown in Table 1 was heated to 1300° C. and rolled in a universal rolling mill train shown in FIG. 1 where the rolled material 5 (cast slab) emerging from the heating furnace 1 was passed through a coarse rolling mill 2, intermediate rolling mill 3, and final rolling mill 4 in that order to be rolled into, as shown in FIG. 2, H-section steel having an H-shaped cross-section comprising a web 6 and a pair of flanges 7 (H458×417×30×50). Note that the rolling heating temperature was standardized at 1300° C. because in general the fact that a drop in the heating temperature makes the γ grains finer and improves the mechanical properties is well known, high temperature heating conditions are estimated to give the lowest values of the mechanical properties, and this value is judged to be able to represent the properties of the heating temperature below it.

Each of the thus produced H-section steels had test pieces taken at positions at the center parts of plate thickness t₂ of the flange 7 (½t₂) and ½ the width of the flange width as a whole (B) (½B) and examined for mechanical properties. Note that the properties of this location were examined because the ½B part of the flange drops most in mechanical properties of the H-section steel, so it was judged possible to determine the mechanical test properties of the H-section steel from this location.

Table 2 shows, as mechanical test properties of H-section steel produced from various steel types, the yield strength at 600° C. (600° C.PS (MPa)), the yield strength (yield point stress YP (MPa)) and tensile strength (TS (MPa)) at ordinary temperature (21° C.), the ratio of the 0.2% yield strength at 600° C. (600° C.PS) and the yield strength (yield point stress YP) at ordinary temperature (21° C.) (600° C.PS/YP ratio (%)), the yield ratio (YR), the impact value (vE0° C. (J)), and the brittle fracture rate (%). Note that as the passing standards of the mechanical test properties, a tensile strength TS at ordinary temperature (21° C.) of 590 MPa or more, a yield strength (YP) of a high strength of 440 MPa or more, a 0.2% yield strength at 600° C. of 2.3 of the 440 MPa lowest standard of the yield strength at ordinary temperature (21° C.) (293 MPa) or more, a yield strength at 600° C. of 60% or more of the yield strength at 21° C., a yield ratio YR of 80% or less, an impact value vE0° C. of 70 J or more, and a brittle fracture rate of 50% or less are demanded. With these passing standards, the standards of the Architectural Institute of Japan can be cleared and the steel can be judged as suitable as fire resistant steel.

TABLE 2 600° C. Impact Brittle 600° C. PS/YP value fracture Steel PS YP TS ratio YR vE0° C. rate type MPa MPa MPa % % J % Remarks Inv. 1 393 545 682 72.1 79.9 388  3 steel 2 385 510 648 75.5 78.7 390  8 3 387 516 645 75.0 80.0 253  47 4 375 530 663 70.8 79.9 380  7 5 365 508 635 71.9 80.0 403  0 6 311 455 592 68.4 76.9 345  7 7 309 462 591 66.9 78.2 286  17 8 315 468 591 67.2 79.4 198  17 9 331 465 595 71.2 78.2 204  0 10 368 505 635 72.9 79.5 253  0 11 385 520 652 74.0 79.8 396  0 12 385 526 658 73.2 79.9  96  33 13 388 538 665 72.1 80.9 145  47 14 332 449 607 73.9 74.0 209  0 15 392 542 678 72.3 79.9 404  0 16 387 536 673 72.2 79.6 395  0 17 392 541 678 72.5 79.8 398  0 Comp. 18 211 398 488 53.0 81.6 115  33 steel 19 249 379 536 65.7 70.7  58  87 20 380 535 669 71.0 80.0  35  93 21 286 424 543 67.5 78.1 121  47 22 392 530 667 74.0 79.5  12 100 23 291 458 615 63.5 74.5 304  33 24 379 638 842 59.4 75.8  57  93 25 369 503 631 73.4 79.7  98  33 High temp. cracking 26 395 528 667 74.8 79.2  79  47 High temp. cracking 27 302 521 655 58.0 79.5 422  0 28 296 522 659 56.7 79.2 403  0 29 345 536 698 64.4 76.8  23 100 30 383 525 682 73.0 77.0  43 67 31 361 533 671 67.7 79.4  37 100 32 249 388 551 64.2 70.4 377  3 33 268 397 515 67.5 77.1 224 17 34 386 549 687 70.3 79.9  25  53 35 368 521 656 70.6 79.4  15  93 36 286 468 548 61.1 85.4 380  3 37 243 390 539 62.3 72.4 212  7 38 378 538 704 70.3 76.4  17  93

The H-section steels produced by the Steel Types 1 to 17 in the range of the present invention all were able to clear the above passing standards. As opposed to this, the Steel Types 18 to 38 (comparative steels) outside the range of the present invention failed to clear the above passing standards even partially. In particular, it was learned that Steel Types 27 and 28 having Ni/Cu ratios over 0.9 had yield strengths at 600° C. of less than 60% of the yield strengths at 21° C. Further, Steel Types 25 and 26 having Ni/Cu ratios less than 0.6 had high temperature cracking at the time of rolling.

Here, the range of the present invention (optimum range) determined by the range of the Ni/Cu ratio (in the present invention, 0.6 to 0.9) and the ratio of the yield strength at 600° C. (high temperature PS) and yield strength at 21° C. (ordinary temperature YP) (in the present invention, 60% or more) is shown in FIG. 3. Note that Steel Types 27 and 28 and Steel Types 25 and 26 outside the range of the present invention were entered in FIG. 3.

Table 3 shows the mechanical test properties in the case of changing the average cooling rate in the temperature range of 800 to 500° C. after the end of rolling for the Steel Type 2 of Table 1. The Test Pieces 1 to 3 with an average cooling rate of the temperature range of 800 to 500° C. after rolling all could clear the passing standards. As opposed to this, Test Piece 4 of the comparative example with an average cooling rate of the temperature range of 800 to 500° C. after rolling of less than 0.1° C./s had a small cooling rate, so α-structures were formed in large amounts before the bainite transformation and therefore the yield ratio fell and the passing standards could not be satisfied.

TABLE 3 600° C. Impact Brittle Test Cooling 600° C. PS/YP value fracture piece Steel rate PS YP TS ratio YR vE0° C. rate no. type ° C./s MPa MPa MPa % % J % Remarks 1 2 1 379 505 651 75.0 77.6 366 0 Inv. ex. 2 2 0.2 385 510 648 75.5 78.7 390 8 Inv. ex. 3 2 0.1 329 498 626 66.1 79.6 98 33 Inv. ex. 4 2 0.05 301 486 591 61.9 82.2 79 47 Comp. ex.

The H-section steels in the range of the present invention had sufficient ordinary temperature and high temperature strengths and were superior in fire resistance and toughness even at parts of ½ the flange thickness and ½ the width where the mechanical test properties of rolled section steel are hardest to guarantee. Note that the examples verified the results for H-section steel, but the rolled steel materials covered by the present invention are not limited to the H-section steel of the examples. The invention can also be applied to I-section steel, angle steel, channel steel, unequal side, unequal thickness angle steel, and various other types of section steel, thick-gauge plate and other such steel plate, etc. of course.

INDUSTRIAL APPLICABILITY

The present invention can be utilized for fire resistant steel etc. used for example for structural members of buildings. 

1. Fire resistant steel characterized by containing, by mass %, C: 0.01 to 0.03%, Mn: 0.2 to 1.7%, Si: 0.5% or less, Cu: 0.7 to 2%, Mo: 0.8% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, N: 0.006% or less, B: 0.0003 to 0.003%, V: 0.2% or less, Cr: 1% or less, Al: 0.1% or less, P: 0.03% or less, and S: 0.02% or less, containing Ni by mass ratio of Ni/Cu of 0.6 to 0.9, and comprising a balance of Fe and unavoidable impurities, and having a yield strength at 600° C. of 60% of the yield strength at 21° C.
 2. Fire resistant steel as set forth in claim 1 characterized by further containing, by mass %, one or more of any of Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%, and comprising a balance of Fe and unavoidable impurities.
 3. Fire resistant steel as set forth in claim 1 or 2 characterized in that said steel material is section steel.
 4. A method of production of fire resistant steel characterized by heating a steel slab containing, by mass %, C: 0.01 to 0.03%, Mn: 0.2 to 1.7%, Si: 0.5% or less, Cu: 0.7 to 2%, Mo: 0.8% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, N: 0.006% or less, B: 0.0003 to 0.003%, V: 0.2% or less, Cr: 1% or less, Al: 0.1% or less, P: 0.03% or less, and S: 0.02% or less, containing Ni by mass ratio of Ni/Cu of 0.6 to 0.9, and comprising a balance of Fe and unavoidable impurities to a temperature range of 200 to 1350° C., then starting rolling and, after finishing rolling, cooling by a cooling rate of an average of 0.1° C./s or more in the temperature range of 800° C. to 500° C.
 5. A method of production of fire resistant steel as set forth in claim 4, characterized by heating a steel slab further containing, by mass %, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01% and comprising a balance of Fe and unavoidable impurities to a temperature range of 1200 to 1350° C., then starting rolling and, after finishing rolling, cooling by a cooling rate of an average of 0.1° C./s or more in the temperature range of 800° C. to 500° C.
 6. A method of production of fire resistant steel as set forth in claim 4 or 5 characterized by rolling to produce section steel. 